Method for fabricating MEMS device integrated with a semiconductor integrated circuit

ABSTRACT

A method for fabricating a semiconductor device is disclosed. A semiconductor substrate comprising a MOS transistor is provided. A MEMS device is formed over the MOS transistor. The MEMS device includes a bottom electrode in a second topmost metal layer, a diaphragm in a pad metal layer, and a cavity between the bottom electrode and the diaphragm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 15/373,489 filed Dec. 9, 2016, which is included in its entirety herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to the field of semiconductor technology. More particular, the present invention relates to a semiconductor device with integrated MEMS (Micro-Electro-Mechanical Systems) structures, and a method for fabricating the same.

2. Description of the Prior Art

Micro-Electro-Mechanical Systems, or MEMS, is a technology that in its most general form can be defined as miniaturized mechanical and electro-mechanical elements (i.e., devices and structures) that are made using the techniques of microfabrication.

As known in the art, poly silicon is typically used as a mechanical layer in fabricating MEMS devices because of its mechanical, electrical, and thermal properties. However, the fabrication techniques used for polysilicon-based MEMS devices are complex and are not compatible with the complementary metal-oxide semiconductor (CMOS) processes.

There is an unmet need in this industry for integrating MEMS devices with a semiconductor integrated circuit die, and a method for fabricating the MEMS device that is fully compatible with CMOS processes.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a semiconductor device includes a semiconductor substrate comprising a MOS transistor. A MEMS device is integrally constructed above the MOS transistor. The MEMS device includes a bottom electrode in a second topmost metal layer, a diaphragm in a pad metal layer, and a cavity between the bottom electrode and the diaphragm. The MEMS device may be a microphone or a pressure sensor.

According to another aspect of the invention, a semiconductor device includes a semiconductor substrate having a plurality of metal layers thereon. A first MEMS device is integrally constructed in the plurality of metal layers on the semiconductor substrate. The first MEMS device has a first cavity. A second MEMS device is integrally constructed in the plurality of metal layers on the semiconductor substrate. The second MEMS device has a second cavity. A volume of the second cavity is greater than that of the first cavity.

According to still another aspect of the invention, a method for fabricating a semiconductor device is disclosed. A semiconductor substrate comprising a MOS transistor is provided. A MEMS device is formed over the MOS transistor. The MEMS device includes a bottom electrode in a second topmost metal layer, a diaphragm in a pad metal layer, and a cavity between the bottom electrode and the diaphragm.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the embodiments, and are incorporated in and constitute a part of this specification. The drawings illustrate some of the embodiments and, together with the description, serve to explain their principles. In the drawings:

FIG. 1 to FIG. 6 are schematic, cross-sectional diagrams showing a method for fabricating a semiconductor device with MEMS structures integrally constructed in the topmost metal layer, the second topmost metal layer, and the pad metal layer according to one embodiment of the invention;

FIG. 7 is an exemplary layout diagram showing the arrangement of the bottom electrode, the diaphragm, and via opening for forming the cavity between the bottom electrode and the diaphragm; and

FIG. 8 is a system block diagram according to another embodiment of the invention.

DETAILED DESCRIPTION

In the following detailed description of the disclosure, reference is made to the accompanying drawings, which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural changes may be made without departing from the scope of the present disclosure.

The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

One or more implementations of the present invention will now be described with reference to the attached drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the illustrated structures are not necessarily drawn to scale.

The terms wafer and substrate used herein include any structure having an exposed surface onto which a material layer is deposited, for example, to form a circuit structure including, but not limited to, an interconnection metal line or a redistribution layer (RDL). The term substrate is understood to include semiconductor wafers, but not limited thereto. The term substrate is also used to refer to semiconductor structures during processing, and may include other layers that have been fabricated thereupon.

The present disclosure pertains to integrated CMOS/MEMS die having a MEMS device that is integrally constructed in the topmost metal layer, a second topmost metal layer, and a pad metal layer on a semiconductor substrate. The MEMS device may be fabricated with CMOS compatible processes and may be directly over an active circuit such as a MOS transistor or a CMOS circuit region.

Throughout the specification and drawings, the symbol “M_(n)” refers to the topmost level of the metal layers below an aluminum redistribution layer (RDL), fabricated in the integrated circuit chip, while “M_(n-1)” refers to the second topmost metal layer that is one level lower than the topmost metal layer and so on, wherein, preferably, n ranges between 2 and 10 (n=2−10), but not limited thereto. The symbol “V” refers to the via plug connecting two adjacent levels of the metal layers. For example, V₆ refers to the via plug interconnecting M₆ to M₅.

Please refer to FIG. 1 to FIG. 7. FIG. 1 to FIG. 6 are schematic, cross-sectional diagrams showing a method for fabricating a semiconductor device 1 with MEMS structures integrally constructed in the topmost metal layer, the second topmost metal layer, and the pad metal layer according to one embodiment of the invention. FIG. 7 is an exemplary layout diagram showing the arrangement of the bottom electrode, the diaphragm, and via opening for forming the cavity between the bottom electrode and the diaphragm.

As shown in FIG. 1, a semiconductor substrate 100 is provided. The semiconductor substrate 100 may comprise single crystalline silicon, SiGe, or epitaxial silicon, but is not limited thereto. At least a metal-oxide-semiconductor (MOS) transistor 101 is fabricated on a major surface 100 a of the semiconductor substrate 100. A plurality of dielectric layers including inter-metal dielectric (IMD) layers 201, 203, and 205 are formed on the semiconductor substrate 100. A plurality of metal layers may be fabricated in the plurality of dielectric layers.

For example, a topmost metal layer M_(n) may be fabricated in the IMD layer 205, a second topmost metal layer M_(n-1) may be fabricated in the IMD layer 201, and a via layer V_(n) may be fabricated in the IMD layer 203. According to one embodiment, the topmost metal layer M_(n), the second topmost metal layer M_(n-1), and the via layer V_(n) may be copper damascene structures.

The aforesaid dielectric layers on the semiconductor substrate 100 may comprise silicon oxide, silicon nitride, silicon oxy-nitride, ultra-low k dielectric, or any suitable dielectric materials known in the art. For example, the IMD layers 201, 203, 205 may comprise fluorosilicate glass (FSG). A silicon nitride cap layer 202 may be formed between the IMD layer 201 and the IMD layer 203. A silicon oxy-nitride stop layer 204 may be formed between the IMD layer 203 and the IMD layer 205.

It is understood that the dielectric stack shown in the figures are for illustration purposes only. Other dielectric structures or materials may be employed without departing from the scope of the present invention.

According to one embodiment, a bottom electrode 210 is fabricated in the second topmost metal layer M_(n-1). According to one embodiment, optionally, the bottom electrode 210 may be connected to a first via 213 a and a second via 213 b. The first via 213 a and second via 213 b may be fabricated in the IMD layer 203. According to one embodiment, optionally, the first via 213 a and second via 213 b may be further connected to a first metal pattern 215 a and a second metal pattern 215 b, respectively. The first metal pattern 215 a and the second metal pattern 215 b may be fabricated in the IMD layer 205.

According to one embodiment, a sacrificial metal pattern 215 c is fabricated in the topmost metal layer M_(n) and embedded in the IMD layer 205 between the first metal pattern 215 a and the second metal pattern 215 b. According to one embodiment, the sacrificial metal pattern 215 c may be surrounded by the first metal pattern 215 a and the second metal pattern 215 b and may be spaced apart from the first metal pattern 215 a and the second metal pattern 215 b.

According to one embodiment, the sacrificial metal pattern 215 c may be isolated from the first metal pattern 215 a and the second metal pattern 215 b by the IMD layer 205. In another embodiment, the sacrificial metal pattern 215 c may be contiguous with the first metal pattern 215 a and the second metal pattern 215 b.

According to one embodiment, a dielectric layer 310 is then deposited on the IMD layer 205 and on the topmost metal layer M_(n) including the sacrificial metal pattern 215 c. According to one embodiment, the dielectric layer 310 may comprise silicon oxide or silicon nitride, but is not limited thereto. For example, the dielectric layer 310 may comprise a TEOS (tetraethylorthosilicate) oxide layer and a silicon nitride layer on the TEOS oxide layer. Subsequently, a lithographic process and an etching process are performed to form an opening 310 a in the dielectric layer 310.

According to one embodiment, the opening 310 a is completely overlapped with the sacrificial metal pattern 215 c when viewed from the above. According to one embodiment, the opening 310 a partially exposes a top surface of the underlying sacrificial metal pattern 215 c.

As shown in FIG. 2, after the formation of the opening 310 a in the dielectric layer 310, a pad metal layer 320 is formed on the dielectric layer 310 and in the opening 310 a. According to one embodiment, the pad metal layer 320 may comprise aluminum or aluminum-copper alloys, but is not limited thereto. The pad metal layer 320 may be patterned by conventional lithographic and dry etching processes.

According to one embodiment, the pad metal layer 320 may comprise a diaphragm pattern 320 a situated within the opening 310 a. According to one embodiment, the diaphragm pattern 320 a is in direct contact with the sacrificial metal pattern 215 c. According to one embodiment, the diaphragm pattern 320 a does not completely fill the opening 310 a. A gap (not shown in this figure) may be formed between an edge of the diaphragm pattern 320 a and the dielectric layer 310.

According to one embodiment, the pad metal layer 320 may further comprise a pad pattern 320 b for forming a bump pad. According to one embodiment, the pad pattern 320 b is situated directly on the dielectric layer 310 and may be electrically connected to the diaphragm pattern 320 a. According to one embodiment, the pad metal layer 320 and the dielectric layer 310 constitute a re-distribution layer (RDL) structure 300 over the topmost metal layer M_(n).

As shown in FIG. 3, after the formation of the pad metal layer 320 and the diaphragm pattern 320 a in the opening 310 a, a passivation layer 340 is deposited over the semiconductor substrate 100. According to one embodiment, the passivation layer 340 may comprise silicon nitride, silicon oxide, polyimide, or any suitable passivation materials known in the art. The passivation layer 340 conformally covers the dielectric layer 310 and the pad metal layer 320 including the diaphragm pattern 320 a. The passivation layer 340 also fills into the gap between an edge of the diaphragm pattern 320 a and the dielectric layer 310.

As shown in FIG. 4, a lithographic process and a dry etching process are performed to form at least a via opening 340 a penetrating through the passivation layer 340. According to one embodiment, the via opening 340 a may be situated directly above the gap between an edge of the diaphragm pattern 320 a and the dielectric layer 310. According to one embodiment, the via opening 340 a may be self-aligned to the edge of the diaphragm pattern 320 a and may partially expose the edge of the diaphragm pattern 320 a.

Please refer briefly to FIG. 7. FIG. 7 is an exemplary layout diagram showing the arrangement of the bottom electrode 210, the sacrificial metal pattern 215 c, the diaphragm 320 a (pad metal layer 320), the opening 310 a, and the via opening 340 a. According to one embodiment, the via opening 340 a may be situated directly above the gap 360 between an edge of the diaphragm pattern 320 a and the dielectric layer 310. According to one embodiment, the via opening 340 a may be self-aligned to the edge of the diaphragm pattern 320 a and may partially expose the edge or a sidewall surface of the diaphragm pattern 320 a.

According to one embodiment, a pad opening 340 b may be formed in the passivation layer 340 directly above the pad pattern 320 b. After the formation of the via opening 340 a and the pad opening 340 b, a selective wet etching process is performed to etching away the sacrificial metal pattern 215 c through the via opening 340 a, thereby forming a cavity 400 between the diaphragm 320 a and the bottom electrode 210. For example, the selective wet etching process may involve the use of EKC solution or solvent that is able to selectively remove the copper.

According to one embodiment, the cavity 400 is capped by the diaphragm 320 a, the dielectric layer 310, and the passivation layer 340. The via opening 340 a communicates with the cavity 400. The diaphragm 320 a, the bottom electrode 210, and the cavity 400 constitute a MEMS device 20, which may function as a microphone or a pressure sensor, but is not limited thereto.

As shown in FIG. 6, after the formation of the cavity 400, an under-bump-metallurgy (UBM) layer 410 may be formed in the pad opening 340 b. The UBM layer 410 may comprise nickel, gold, titanium, or the like. Subsequently, a bump pad 420 such as a copper layer is formed on the UBM layer 410. According to one embodiment, a solder bump 430 may be formed on the bump pad 420.

Please refer to FIG. 8. FIG. 8 is a system block diagram according to another embodiment of the invention. According to the embodiment, the semiconductor device 2 has two MEMS devices 20 a and 20 b fabricated on a semiconductor substrate. The structure and the fabrication steps of the two MEMS devices 20 a and 20 b are similar to that as shown through FIG. 1 to FIG. 7. According to the embodiment, the two MEMS devices 20 a and 20 b are two MEMS microphones with cavities of different dimensions for decoupling noise.

As shown in FIG. 8, and briefly back to FIG. 1 to FIG. 7, likewise, the semiconductor device 2 comprises a semiconductor substrate 100 having thereon a plurality of metal layers. A first MEMS device 20 a is integrally constructed in the plurality of metal layers on the semiconductor substrate 100. The first MEMS device 20 a comprises a first cavity. A second MEMS device 20 b is integrally constructed in the plurality of metal layers on the semiconductor substrate 100. The second MEMS device 20 b comprises a second cavity. A volume of the second cavity is greater than that of the first cavity.

According to the embodiment, the second MEMS device 20 b is a low-frequency microphone and the first MEMS device 20 a is a high-frequency microphone. The first MEMS device 20 a may be electrically coupled to a first pre-amplifier 510, and the second MEMS device 20 b may be electrically coupled to a second pre-amplifier 520. The first pre-amplifier 510 and the second pre-amplifier 520 are both coupled to a mixer 620. The mixer 620 may be coupled to an Application-Specific Integrated Circuit (ASIC) 700.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A method for fabricating a semiconductor device, comprising: providing a semiconductor substrate comprising a metal-oxide-semiconductor (MOS) transistor; and forming a Micro-Electro-Mechanical Systems (MEMS) device over the MOS transistor, wherein the MEMS device comprising a bottom electrode in a second topmost metal layer, a diaphragm in a pad metal layer, and a cavity between the bottom electrode and the diaphragm, wherein the bottom electrode is a continuous planar structure; forming a first inter-metal dielectric (IMD) layer on the semiconductor substrate; forming the second topmost metal layer in the first IMD layer; forming a second IMD layer on the first IMD layer; forming a topmost metal layer in the second IMD layer; forming a dielectric layer covering the topmost metal layer and the second IMD layer; and forming an opening in the dielectric layer, wherein the opening partially exposes a top surface of the topmost metal layer; forming the pad metal layer on the dielectric layer and in the opening; and forming a passivation layer on the pad metal layer; forming a via opening penetrating through the passivation layer and partially exposing the top surface of the topmost metal layer; and etching away the topmost metal layer through the via opening, thereby forming the cavity.
 2. The method for fabricating a semiconductor device according to claim 1, wherein the topmost metal layer is etched away through the via opening by using a wet etching method.
 3. The method for fabricating a semiconductor device according to claim 1 further comprising: forming a pad opening in the passivation layer above the pad metal layer; forming an under-bump-metallurgy (UBM) layer in the pad opening; forming a bump pad on the UBM layer; and forming a bump on the bump pad. 